If you want to create something for Cosmic Cutthroats … nothing is stopping you!
The sections of Cosmic Cutthroats that contain only game mechanical material are released under the Creative Commons Attribution Sharealike license.
What does this mean? It means you are free to create your own material using the R.E.C.I.P.E. game engine! Create new Realms, NPCs, and Adventures, and share them with others, even for sale!
The specific terms are in Section 1.3 of Cosmic Cutthroats, but the short version is: (1) You can use the game mechanics of Cosmic Cutthroats, but no Realms, settings, or named Characters in the game or any later supplements, (2) you do not have license from me to use any of the art in Cosmic Cutthroats, (3) please credit Cosmic Cutthroats as the original ruleset, and me as its author.
For more details on this Creative Commons license:
I have the utmost faith in the game community not to use these game materials for hate speech, bigotry, or discrimination of any kind. Please don’t let me down!
With that in mind, go for it, and let’s see what amazing worlds you create!